Is anybody else having trouble with this achievement? I have completed round 3 three times without taking damage and still no achievement! I love this game and would love to get all the achievements. My wife plays with me and I kite around the hard enemies while she picks them off. Is it okay for your partner to take damage? Do I have to do this one solo? I'm confused. Any feedback from anyone with similar problems or any clarification on how to get this one would be great. Thanks!

The achievement is awarded only to the locally controlled heroes (no remotely in case of an online game). The achievement is no different than the "Elite Gladiator". If you have it, then there shouldn't be a problem with the other one. I guess that if you play coop game on a split screen, then none of you should take damage in order to gain the achievement.